Dislocation-governed current-transport mechanism in (Ni/Au)–AlGaN/AlN/GaN heterostructures

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Dislocation-governed current-transport mechanism in (Ni/Au)–AlGaN/AlN/GaN heterostructures

Engin Arslan, Şemsettin Altındal, Süleyman Özçelik, and Ekmel Ozbay

Citation: J. Appl. Phys. 105, 023705 (2009); doi: 10.1063/1.3068202 View online: http://dx.doi.org/10.1063/1.3068202

View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v105/i2 Published by the American Institute of Physics.

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Dislocation-governed current-transport mechanism in „Ni/Au…–AlGaN/AlN/

GaN heterostructures

Engin Arslan,^1,a兲 Şemsettin Altındal,²Süleyman Özçelik,²and Ekmel Ozbay³

1Nanotechnology Research Center-NANOTAM, Bilkent University, 06800 Ankara, Turkey

2Department of Physics, Faculty of Science and Arts, Gazi University, Teknikokullar, 06500 Ankara, Turkey

3Department of Physics, Department of Electrical and Electronics Engineering, and Nanotechnology Research Center-NANOTAM, Bilkent University, 06800 Ankara, Turkey

共Received 31 July 2008; accepted 4 December 2008; published online 22 January 2009兲

The current-transport mechanisms in 共Ni/Au兲–Al0,22Ga_0,78N/AlN/GaN heterostructures were studied by using temperature dependent forward-bias current-voltage 共I-V兲 characteristics in the temperature range of 80–410 K. In order to determine the current mechanisms for 共Ni/Au兲–Al0,22Ga_0,78N/AlN/GaN heterostructures, we fitted the experimental I-V data to the analytical expressions given for the current-transport mechanisms in a wide range of applied biases and at different temperatures. The contributions of thermionic-emission, generation-recombination, tunneling, leakage currents that are caused by inhomogeneities, and defects at the metal-semiconductor interface current mechanisms were all taken into account. The best fitting results were obtained for the tunneling current mechanism. On the other hand, we did not observe sufficient agreement between the experimental data and the other current mechanisms. The temperature dependencies of the tunneling saturation current共It兲 and tunneling parameters 共E₀兲 were obtained from fitting results. We observed a weak temperature dependence of the saturation current and the absence of the temperature dependence of the tunneling parameters in this temperature range. The results indicate that in the temperature range of 80–410 K, the mechanism of charge transport in the共Ni/Au兲−Al0.22Ga_0.78N/AlN/GaN heterostructure is performed by tunneling among those dislocations intersecting the space charge region. The dislocation density 共D兲 that was calculated from the I-V characteristics, according to a model of tunneling along the dislocation line, gives the value of 0.24⫻10⁷ cm⁻². This value is close in magnitude to the dislocation density that was obtained from the x-ray diffraction measurements. © 2009 American Institute of Physics.

关DOI:10.1063/1.3068202兴

I. INTRODUCTION

The transport properties of GaN and its alloys are at- tracting increasing interest due to the potential application of these materials for solar-blind photodetectors and high mobility transistors.^1–3 Because of the large energy band gap, the applications of AlxGa_1−xN are extensive, such as for visible-blind ultraviolet 共UV兲 detectors, laser diodes, and short-wave light-emitting diodes.⁴High-quality AlGaN/GaN heterostructures have been shown to contain two- dimensional electron gas 共2DEG兲, which has attracted spe- cial interest due to its potential applications in high mobility transistors operating at high power and high temperature levels.^5,6

The device structures are usually grown on highly lattice-mismatched substrates, such as sapphire,⁷ SiC,⁸ or Si.^3,9 It still remains difficult to obtain a high-quality GaN epilayer because of the large lattice mismatch and large dif- ference in the thermal expansion coefficients between the GaN film, sapphire, and SiC substrate. This fact causes a high level of in-plane stress and threading dislocation density 共DD兲 generation, as grown by metal-organic chemical vapor

deposition 共MOCVD兲 in the GaN epitaxial layer.^7–9 These dislocations affect the performance reliability of the device.^1,2Furthermore, it is well known that GaN usually has a high DD, much more than those in Si and GaAs semiconductors. If there exist many defects near the surface region, the electrons can easy go through the barrier by/via defect- assisted tunneling, thus greatly enhancing the tunneling probability. It is well established that the crystal qualities of Si and GaAs are far superior to that of GaN. For Schottky barriers on Si and GaAs with a doping concentration of

⬃10¹⁶ cm⁻³, tunneling does not play a role in the current- transport mechanisms.

The current-transport mechanism in these devices such as metal semiconductor共MS兲, metal-insulator semiconductor 共MIS兲, and solar cells are dependent on various parameters such as the process of surface preparation, formation of insulator layer between the metal and semiconductor, barrier height inhomogenity, impurity concentration of a semiconductor, density of interface states or defects, series resistance 共Rs兲 of a device, device temperature, and bias voltage. In these devices, a number of carrier transport mechanisms such as quantum mechanical tunneling, thermionic emission共TE兲, thermionic field emission 共TFE兲, minority carrier injection, recombination generation, and multistep tunneling compete, and usually, one of them may dominate over the others in a certain temperature and voltage regions. However, a simul-

a兲Author to whom correspondence should be addressed. Tel.:⫹90-312- 2901971. FAX: ⫹90-312-2901015. Electronic mail:

engina@bilkent.edu.tr.

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taneous contribution from two or more mechanisms could also be possible. Among them, TFE is important at low temperatures and high doping concentration levels. Voltage- proportional leakage currents frequently alter the current- voltage 共I-V兲 relationship especially at lower bias voltage levels. These leakage currents are usually expressed by the addition of a term 共V-IRs兲/Rsh to the total current due to series resistance 共Rs兲 and shunt resistance 共Rsh兲 of device.

More recently, experimental results have been shown for MS, MIS, and solar cells.^10–14Very interesting studies among these were presented by Kar et al.¹⁰and Cao et al.,¹¹where the results indicated the likelihood of a primary current- transport mechanism being multistep tunneling and defect- assisted tunneling instead of TE, respectively. Evstropov et al.^12,13 and Balyaev et al.¹⁴ showed that the current flow in the III–V heterojunctions is generally governed by multistep tunneling with the involvement of dislocations even at room temperature. They demonstrated that an excess tunnel current can be attributed to dislocations. A model of tunneling through a space charge region along a dislocation line共tube兲 is suggested.¹²

Analysis of the current-voltage 共I-V兲 characteristics of the MS, MIS, and solar cells measured only at room temperature does not provide detailed information about the current-conduction process and the nature of the barrier formed at the metal/semiconductor interface. On the other hand, the forward bias I-V characteristics at a wide tempera- ture range enable us to understand the different aspects of the current-conduction mechanism and barrier formation. There- fore, the main aim of this study is to investigate the current- conduction mechanisms in AlGaN/AlN/GaN heterostructures with a high dislocation compared with the literature in a wide temperature range共80–410 K兲.

II. EXPERIMENTAL PROCEDURE

The AlxGa_1−xN/AlN/GaN 共x=0.22兲 heterostructures were fabricated on共0001兲 on a double-polished 2 –in. diameter 共0001兲, in which sapphire 共Al2O₃兲 substrates were grown in a low pressure MOCVD reactor 共Aixtron 200/4 HT-S兲 by using trimethylgallium, trimethylaluminum, and ammonia as Ga, Al, and N precursors, respectively. Prior to the epitaxial growth, Al₂O₃ substrate was annealed at 1100 ° C for 10 min in order to remove surface contamina- tion. The buffer structures consisted of a 15 nm thick, low- temperature 共650 °C兲 AlN nucleation layer, and high temperature 共1150 °C兲 420 nm AlN templates. A 1.5 ␮^m nominally undoped GaN layer was grown on an AlN tem- plate layer at 1050 ° C, followed by a 2 nm thick high temperature AlN共1150 °C兲 barrier layer. The AlN barrier layer was used to reduce the alloy disorder scattering by minimiz- ing the wave function penetration from the 2DEG channel into the AlxGa_1−xN layer.¹⁵After the deposition of these layers, a 23 nm thick undoped Al_0.22Ga_0.78N layer was grown on an AlN layer at 1050 ° C. Finally, a 5 nm thick GaN cap layer growth was carried out at a temperature of 1085 ° C and a pressure of 50 mbars.

Since the sapphire substrate is insulating, the Ohmic and Schottky/rectifier contacts were made bottom and top of the

sample, respectively, in the high coating system at about 10⁻⁷ Torr. The Ohmic contacts were formed as a square van de Pauw shape and the Schottky contacts formed as 1 mm diameter circular dots 共Fig.1兲. Prior to Ohmic contact formation, the samples were cleaned. The samples are cleaned with acetone in an ultrasonic bath. Then, a sample was treated with boiling isopropyl alcohol for 5 min and rinsed in de-ionized共DI兲 water having 18 M⍀ resistivity. After clean- ing, the samples were dipped in a solution of HCl/H2O共1:2兲 for 30 s in order to remove the surface oxides, and then rinsed in DI water again for a prolonged period. Ti/Al/Ni/Au 共17.5/175/40/80 nm兲 metals were thermally evaporated on the sample and were annealed at 850 ° C for 30 s in N₂ ambient in order to form the Ohmic contact. Schottky contacts were formed by Ni/Au共40/80 nm兲 evaporation.

The crystalline quality 共DD兲 of the samples of the samples was examined by high-resolution x-ray diffraction 共HRXRD兲. The x-ray diffraction was performed by using a Bruker D-8 high-resolution diffractometer system, delivering Cu K␣1 共1.540 Å兲 radiation. The current-voltage 共I-V兲 mea- surements were performed by the use of a Keithley 2400 sourcemeter in the temperature range of 80–410 K using a temperature controlled Janes vpf-475 cryostat, which enables us to make measurements in the temperature range of 77–

450 K. The sample temperature was continually monitored by using a copper-constant thermocouple close to the sample and measured with a Keithley model 199 dmm/scanner and a Lake Shore model 321 autotuning temperature controller with sensitivity better than⫾0.1 K.

III. RESULTS AND DISCUSSION

In the present study, the Al_0.22Ga_0.78N/AlN/GaN heterostructures were grown on sapphire with two steps exhibited high DDs.^3,7The DD of the sample was investigated by the methods of high-resolution diffractometry. The DDs in the Al_0.22Ga_0.78N/AlN/GaN heterostructures can be taken as equal to the DDs in the GaN layers.³There are three main types of dislocations that are present in the GaN epitaxial layers.^16,17The pure edge dislocation with Burgers vector b

=¹₃具112¯0典共具a典兲, the pure screw dislocation with Burgers vec-

FIG. 1. 共Color online兲 Schematic of the Al0.22Ga_0.78N/AlN/GaN heterostructure and a view of the Ohmic contact and Schottky contact on the structures.

023705-2 Arslan et al. J. Appl. Phys. 105, 023705共2009兲

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tor b =具0001典共具c典兲, and the mixed dislocation with b

=¹₃具112¯3典共具c+a典兲. These DDs of GaN can be determined from the following equations:¹⁶

D_screw= ␤_共002兲²

9b_screw² , D_edge= ␤_共121兲²

9b_edge² , 共1兲

D_dis= D_screw+ D_edge, 共2兲

where D_screwis the screw DD, D_edgeis the edge DD,␤^{is the} full width at half maximum共FWHM兲 of the measured XRD rocking curves, and b is the Burgers vector length 共bscrew

= 0.5185 nm, b_edge= 0.3189 nm兲. The measured XRD rocking curves for共002兲 and 共121兲 reflections are shown in Fig.

2. The FWHM values measured for共002兲 and 共121兲 reflections are 122.4 and 374.4 arc sec, respectively. By using Eqs. 共1兲 and共2兲, we calculated the values for screw, edge, and total DDs as 1.45⫻10⁷, 2 , 41⫻10⁸, and 2.55

⫻10⁸ cm⁻², respectively.

In order to correctly interpret the current-transport mechanisms in the 共Ni/Au兲–Al0,22Ga_0,78N/AlN/GaN heterostructures, we considered the contribution of TE current and various other current-transport mechanisms. Thus, the generation recombination, tunneling, and leak currents caused by the inhomogenities and defects at the M/S interface were taken into full account. In general, the relationship between the applied-bias voltage共Vⱖ3kT/q兲 and the current of the Schottky diodes, based on the TE theory, is given by¹⁸

I_thermionic= I₀exp

冋

^{q共V − IR}^nkT ^s^兲

册再

¹

− exp

冋

^{− q}^{共V − IR}^kT ^s^兲

册冎

^, ^共3兲

where I₀ is the saturation current derived from the straight- line intercept of ln I at zero bias and is given by

I₀= AA^ⴱT²exp

冉

⁻^q⌽^kT^b0

冊

^, ^共4兲

where A is the rectifier contact area, A^ⴱ is the effective Ri- chardson constant 共32.09 A/cm²K² for undoped Al_0,22In_0,78N兲,¹⁹T is the absolute temperature in kelvins, q is

the electron charge, ⌽B0 is the zero-bias apparent Schottky barrier height, n is the ideality factor, V is the applied-bias voltage, and IR_s term is the voltage drop across the R_s of structure. The generation-recombination current can be de- scribed by the relation²⁰

I_g−r= Igr

再

^exp

^冋

^{q共V − IR}^2kT ^s^兲

^册

^{− 1}

冎

^共5兲

with

Igr=qniWd

2␶ ^, ^共6兲

where I_gris the generation-recombination saturation current, n_i is the intrinsic carrier concentration, W_d is the depletion layer width, and ␶ is the effective carrier lifetime. The tunneling current through the barrier is given by¹⁸

I_tunnel= I_t

再

^exp

^冋

^q^{共V − IR}^E⁰ ^s^兲

^册

^{− 1}

冎

^, ^共7兲

where It is the tunneling saturation current and E₀ is the tunneling parameter. E₀can be defined as^21–23

E₀= E₀₀coth

冉

^EkT⁰⁰

冊

^, ^共8兲

where E₀₀ is the characteristic tunneling energy that is related to the tunnel effect transmission probability. It is evi- dent that the mechanism of charge transport is tunnel, which is indicated by a weak temperature dependence of the saturation current. In addition, voltage-proportional leakage currents frequently alter the current-voltage共I-V兲 relationship at lower bias voltage levels and can be expressed as²¹

I_leakage=V − IRs

R_sh . 共9兲

Thus, the total current of device can be expressed as I_total= I_thermionic+ I_g−r+ I_tunnel+ I_leakage. 共10兲 Figure 3 shows a set of semilogarithmic forward bias I-V characteristics of a 共Ni/Au兲−Al0.22Ga_0.78N/AlN/GaN heterostructure that was measured in the temperature range of 80–410 K. As shown in Fig.3, the forward bias of structure

FIG. 2. The x-ray rocking curves for共002兲 and 共121兲 reflections of the GaN epilayer.

FIG. 3.共Color online兲 Experimental forward and reverse bias semilogarithmic current-voltage characteristics of a共Ni/Au兲–Al0.22Ga_0.78N/AlN/GaN heterostructure at different temperatures.

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current is an exponential function of the applied-bias voltage in the intermediate voltage regime 共0.1ⱕVⱕ0.6 V兲. It is clear that over a broad range共10⁻⁶− 10⁻⁴ A兲 of forward current, the behavior is exponential and beyond that 共I ⱖ10⁻⁴ A兲 the plots deviate from this behavior due to the effect of series resistance共Rs兲. Moreover, these plots are par- allel over a forward-current range of 10⁻⁶− 10⁻⁴ A. To interpret the observed electrical characteristics of an 共Ni/Au兲

− Al_0.22Ga_0.78N/AlN/GaN heterostructure, Riben and Feucht²³ developed a multistep recombination-tunneling model. This model successfully explains the functional dependence of the forward current on applied voltage and temperature. The model, assuming a staircase path that consists of a series of tunneling transitions between trapping levels in the diode space charge region coupled with a series of verti- cal steps where the carrier loses energy by transferring from one level to another. However, such a process is only possible if the concentration of the trapping levels is sufficiently high. In this model, the carrier tunneling between the defect levels increases the probability of tunneling through the en- tire barrier.

The temperature dependent values of zero-bias barrier height 共⌽b0兲 and ideality factor 共n兲, as calculated from the semilogarithmic forward bias ln I versus V characteristics, are shown in Fig.4and TableI. As can be seen in Fig.4and TableI, the values of ⌽b0increase with increasing temperature, in which there is a positive coefficient that is contrary to the negative dependence measurements by Crowell and Rideout²⁴in silicon Shottky diodes and Mead and Spitzer²⁵ in InAs and InSb, which closely follows the charge in for- bidden energy band gap共Eg兲 with temperature. This contra- diction is possible due to Eq.共4兲, which is not representative of the reverse saturation current of our samples implying that the current transport is not the TE.

For the tunneling dominated current-transport equation 共7兲, the slope of the ln I versus V plot 共q/E0= q/nkT兲 is essentially temperature independent and is called a voltage factor or tunneling constant. In addition, at a constant bias voltage, ln I is more of a linear function of temperature than an inverse temperature. According to the tunneling model, which was developed for Schottky barriers, the band bending works as a barrier for carriers tunneling into interface states or dislocations, where various traps may be involved in mul- titunneling steps.²³ Thermally activated carriers make共step- wise兲 tunneling into the interface states. The values of slope ln I versus V plots at different temperatures with the corre- sponding values of the ideality factor关obtained from Eq.共3兲兴 are shown in TableIand Fig.4. As can be seen in TableI, the n values changes from 12.68 共at 80 K兲 to 2.44 共at 410 K兲.

However, the slope and nT values remain almost unchanged over the same temperature range with an average of 11.35 V⁻¹and 1025 K, respectively. The high value of n has been attributed to several effects such as interface states, tunneling currents in the high dislocations,^12–14 image force lowering of the Schottky barrier in the high electric field at a MS interface, and generation currents within the space- charge region.¹⁸The TFE mechanism can be ruled out in this region, since nT is more or less constant in the measured temperature range. Apart from discussing the main carrier transport mechanisms, the ideality factor is further analyzed

FIG. 4. The zero-bias barrier height ⌽b0 and the ideality factor n of a 共Ni/Au兲−Al0.22Ga_0.78N/AlN/GaN heterostructure obtained from the for- ward bias I-V data at various temperatures. The straight line is the least- squares fit of Eq.共8兲to the ideality factor共n兲 data.

TABLE I. Temperature dependent values of various parameters determined from the forward bias I-V charac- teristics of共Ni/Au兲–Al0.22Ga_0.78N/AlN/GaN heterostructure. In the eighth and ninth columns, the values of tunneling saturation current共It兲 and tunneling parameters 共E0兲, obtained by least-squares fit of tunneling current mechanism关Eq.共7兲兴 to the measured I-V data, are listed.

T 共K兲

I₀ x10⁻⁷ A

Slope

共A V⁻¹兲 n

nT 共K兲

E₀

共eV兲 ⌽b0

共eV兲 E₀ 共eV兲

It

x10⁻⁷ A

80 1.84 11.43 12.68 1014.59 0.087 0.16 0.092 3.14

115 1.44 12.36 8.16 938.33 0.081 0.24 0.093 3.19

150 1.79 12.00 6.44 966.04 0.083 0.31 0.096 3.64

185 2.52 11.68 5.37 992.73 0.086 0.39 0.095 4.27

220 2.56 11.79 4.47 982.63 0.085 0.47 0.096 4.51

255 2.30 12.15 3.74 954.53 0.082 0.55 0.096 4.45

290 3.46 11.55 3.46 1003.46 0.086 0.63 0.094 5.00

310 4.26 11.13 3.36 1041.96 0.090 0.66 0.096 5.53

330 4.37 10.99 3.19 1054.58 0.091 0.71 0.096 6.08

350 4.26 11.19 2.96 1035.69 0.089 0.76 0.098 6.38

370 9.28 9.70 3.23 1194.78 0.103 0.78 0.096 6.62

390 8.68 10.06 2.95 1152.02 0.099 0.83 0.096 6.66

410 1.07 11.57 2.44 1002.09 0.086 0.86 0.097 7.03

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by plotting nkT/q against kT/q as shown in Fig. 5, which shows the experimental and theoretical results of these plots.

If the FE dominates, then the E₀ data will lay on a straight line as can be seen in Fig.5. In this case, E₀is independent of the temperature and E₀is very close to the E₀₀values.²⁶In our study, the average value of E₀was found to be 88.5 meV, which is very close to the 85.7 meV value of E₀₀ that was obtained from the fitting of Eq.共8兲to the n共T兲 data 共Fig.4兲.

In order to determine the true current-transport mechanisms for 共Ni/Au兲–Al0,22Ga_0,78N/AlN/GaN heterostruc- tures, by taking the Ite, Igr, It, the tunneling parameter E₀and

the Rsand R_shas adjustable fit parameters, we fit the experi- mental I-V data to the analytical expressions given for the current-transport mechanisms in a wide range of applied biases and at different temperatures. A standard software pack- age was utilized for the curve fitting. As shown in Fig. 6, there is an excellent agreement between the measured I-V data and the current-transport expressions for the tunneling mechanism at all temperature range. However, there is no good fit between the measured I-V data and the analytic ex- pression given for TE, generation recombination, or the leakage current mechanism at all the temperatures 共Fig.6兲. The tunneling saturation current Itand E₀ values, as determined from the fits of the tunneling current expression to the mea- sured I-V data set, are summarized in the eighth and ninth columns of TableI. Similar results have been reported in the literature.11–14,26–29

The temperature dependences of Itand E₀are shown in Fig.7. The results indicate that in the temperature range of 80–410 K, the mechanism of charge transport in the 共Ni/Au兲−Al0.22Ga_0.78N/AlN/GaN heterostructure is tunneling, which is demonstrated by a weak temperature dependence of the saturation current and the absence of the temperature dependence of the tunneling parameters in this temperature range. The I-V behavior of the tunneling current in the barrier structures fabricated based on degenerate semi- conductors共Schottky diodes, p-n heterojunctions兲 can be expressed by Eq.共7兲according to the dislocation model of the tunneling current, and the tunneling saturation current 共It兲 can be represented by the equation of the form^12,14

FIG. 5. Experimentally and theoretically found tunneling current parameter E₀共nkT/q兲 vs kT/q for 共Ni/Au兲–Al0.22Ga_0.78N/AlN/GaN heterostructure.

FIG. 6.共Color online兲 The least-squares fits of the TE 关Eq.共3兲兴, generation recombination 关Eq.共5兲兴, tunneling 关Eq.共7兲兴, and leakage current 关Eq.共9兲兴 equation to the I-V data measured at共a兲 80 K, 共b兲 250 K, 共c兲 330 K, and 共d兲 410 K.

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I_t= qD␯Dexp共− qVK/E0兲, 共11兲 where D is the DD, ␯D⬇1.68⫻10¹³ s⁻¹ is the Debye frequency³⁰ for Al_0.22Ga_0.78N layer and qVK=⌽B−␮n is the diffusion potential for the Schottky barrier diode共SBD兲. In this equation, ⌽B is the height of the SBD, ␮n

⬵kT ln共NC/ND兲 is the chemical potential, Ncis the effective density of states in the conduction band,¹⁸NDis the concentration of the ionized donors in the AlGaN barrier layer, and E₀= nkT is the tunneling parameter.

By using the model equation共11兲and determined Itand E₀from the measured I-V characteristic as well as knowing VK共0兲, which is the expression for the DD that can be given as^12–14

D_dis=It共0兲 q␯D

exp

冋

^qV^E^K⁰^共0兲

册

^, ^共12兲

where I_t共0兲 and E0共0兲 can be obtained by the extrapolation to zero of the absolute temperature of the temperature depen- dences of I_t共0兲 and E0共0兲. The value of qVK共0兲 can be calculated from the empirical dependence of ⌽B on the band gap E_gin GaN 共because in our study Schottky diodes were done on GaN cap layers兲, ⌽B⬇¹₃E_g^GaN,¹⁴

qVK共0兲 = ⌽B共0兲 −␮n共0兲 ⬵¹₃E_g^GaN共0兲 −␮n共0兲. 共13兲 By using It共0兲, E0共0兲, qVK共0兲, and Eg共0兲=3.47 eV values for GaN, the DD can be calculated by Eq.共12兲. In addition, this case was supported by the experimental values of the height of the Schottky barrier that was formed on the GaN layers by pure metals Ni, Pt, Ir,³¹ and Au.³² In the present study, we calculated the DD by using It共0兲=2.31⫻10⁻⁵ A/cm², E₀共0兲=0.092 eV, and qVK共0兲=1.15 eV values. We obtained 0.24⫻10⁷ cm⁻², which are near the results that were obtained from the XRD measurements. The DDs measured from XRD are 1.45⫻10⁷, 2 , 41⫻10⁸, and 2.55⫻10⁸ cm⁻² for the total dislocations, screw-, and edge-type DD, respectively. These DDs are consistent with the values given for GaN epitaxial films in the literature.^3,4,9Analysis of the for- ward bias I-V data indicated that the predominant current mechanism of the Al_0.22Ga_0.78N/AlN/GaN heterostructure with high DDs in the intermediate bias voltage region that was investigated in this study was a dislocation-governed current-transport mechanism rather than the other current- transport mechanisms.

IV. CONCLUSIONS

The current-transport mechanism across 共Ni/Au兲–Al0,22Ga_0,78N/AlN/GaN heterostructures was carried out by using temperature dependent forward-bias current-voltage共I-V兲 characteristics in the temperature range of 80–410 K. The best fitting results were obtained for the tunneling current equation. However, we did not obtain a fit for the other current mechanisms. We did not observe sufficient agreement between the experimental data and the other current mechanisms. The tunneling saturation current共It兲 and tunneling parameter E₀were determined from the fitting process. The typical features of a tunnel saturation current共It兲 temperature dependence were weak and E₀ was nearly independent of the temperature. These results show that the charge transport mechanism in the temperature range of 80–

410 K in the forward-biased 共Ni/Au兲–Al0,22Ga_0,78N/ AlN/GaN heterostructures was performed by the tunneling mechanism among the dislocations intersecting the space charge region.

ACKNOWLEDGMENTS

This work is supported by the European Union under the projects EU-METAMORPHOSE, EU-PHOREMOST, EU- PHOME, and EU-ECONAM, and TUBITAK under the Project Nos. 105E066, 105A005, 106E198, 106A017, and 107A012. One of the authors共E.O.兲 also acknowledges par- tial support from the Turkish Academy of Sciences.

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Dislocation-governed current-transport mechanism in (Ni/Au)–AlGaN/AlN/GaN heterostructures

Dislocation-governed current-transport mechanism in (Ni/Au)–AlGaN/AlN/GaN heterostructures

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Dislocation-governed current-transport mechanism in „Ni/Au…–AlGaN/AlN/

GaN heterostructures

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